This invention relates generally to the measurement of thread pitch diameters and more particularly to a gauge assembly for attachment to an inside micrometer to provide for the accurate measurement of the pitch diameter of an internally threaded part.
When threaded parts are manufactured, it is necessary to accurately measure the pitch diameter of the threads in order to assure that the part has the necessary thread dimensions. Similarly, finished parts must be measured during inspection operations to check the pitch diameter.
The Government has rights in this invention pursuant to Contract No. DE-AC04-76DP00613 awarded by the U.S. Department of Energy.
In my co-pending application Ser. No. 568,837, filed on Jan. 6, 1984, I disclose a thread gauge which can be used to measure the pitch diameter of both external and internal threads. However, the depth to which the gauge can be extended into an internally threaded part is limited by the length of the blades of the caliper instrument to which the gauge is attached. Therefore, even though this device is entirely satisfactory for gauging inside threads on a fairly shallow lip of a part, it is not applicable in situations where the internal threads are located on a recessed area of the part or where the threads are located too deeply in the part to accommodate the caliper blades.
At present, parts which are internally threaded can be measured only with a thread plug gauge which is a go-no go device. The thread plug gauges have standard sizes and are thus able to measure only standard pitch diameters. The thread plug gauge is particularly unsatisfactory if a large number of nonstandard threads are manufactured. In any case, the cost of maintaining a large number of different size plug gauges is often prohibitive, since each gauge is useful only for a single size and is lacking both in versatility and adjustability.
Consequently, it is evident that a need exists for a thread gauge for accurately measuring the thread pitch diameter of internally threaded parts. It is the primary goal of the present invention to meet this need.
More specifically, it is an object of the invention to provide a thread gauge which functions to accurately measure the thread pitch diameter of internally threaded parts having virtually any size and shape. In this respect, it is noteworthy that the thread gauge of the present invention can be applied to any part which can accommodate an inside micrometer, regardless of the location of the threads on the part.
In accordance with the invention, I provide thread anvils having external threads which match the pitch of the internal threads that are to be measured. Each anvil is provided with a bore and a slot which intersects with the bore. Each anvil has a spherical tooling ball which fits closely in its bore. A stem projects from each ball and extends through the slot in the anvil. This arrangement allows each anvil to rotate on its ball so that the micrometer shaft can be turned while the threads on the anvils remain properly oriented to mate with the internal threads of the part which is undergoing measurement. At the same time, the slots permit the anvils to move linearly back and forth on the balls so that the anvil threads can properly align with the threads of the part.
The anvils can be attached to the opposite ends of virtually any inside micrometer from the smallest size to the largest size. A mounting sleeve is fitted on each stem and secured thereto by a set screw. A second set screw is used to lock each sleeve on the micrometer with each stem positioned end to end against the micrometer end. In this manner, the anvils are precisely located on the opposite ends of the micrometer so that the known dimensions of the gauge can be added to the scale reading to thereby provide an accurate measurement of the pitch diameter of the internally threaded part.